G-protein-coupled receptors (GPCRs) comprise a large super-family of integral membrane proteins characterized by having 7 hydrophobic alpha helical transmembrane (TM) domains with three intracellular and three extracellular loops (Ji, et al., J Biol Chem 273:17299-17302, 1998). In addition all GPCRs contain N-terminal extracellular and C-terminal intracellular domains. Binding of extracellular ligand may be mediated by the transmembrane domains, the N-terminus, or extracellular loops, either in alone or in combination. For example binding of biogenic amines such as epinephrine, norepinephrine, dopamine, and histamine is thought to occur primarily at the TM3 site while TM5 and TM6 provide the sites for generating an intracellular signal. Agonist binding to GPCRs results in activation of one or more intracellular heterotrimeric GTP-binding proteins (G proteins) which, in turn, transduce and amplify the signal by subsequent modulation of down-stream effector molecules (such as enzymes, ion channels and transporters). This in turn results in rapid production of second messengers (such as cAMP, cGMP, inositol phosphates, diacylglycerol, cytosolic ions).
GPCRs mediate signal transduction across a cell membrane upon the binding of a ligand to a GPCR. The intracellular portion of the GPCR interacts with a G protein to modulate signal transduction from outside to inside a cell. A GPCR is thus coupled to a G protein. There are three polypeptide subunits in a G-protein complex: an alpha subunit—which binds and hydrolyzes GTP—and a dimeric beta-gamma subunit. In the inactive state, the G protein exists as a heterotrimer of the alpha and beta-gamma subunits. When the G protein is inactive, guanosine diphosphate (GDP) is associated with the alpha subunit of the G protein. When a GPCR is bound and activated by a ligand, the GPCR binds to the G-protein heterotrimer and decreases the affinity of the G alpha subunit for GDP. In its active state, the G subunit exchanges GDP for guanine triphosphate (GTP) and active G alpha subunit disassociates from both the GPCR and the dimeric beta-gamma subunit. The disassociated, active G alpha subunit transduces signals to effectors that are “downstream” in the G-protein signaling pathway within the cell. Eventually, the G protein's endogenous GTPase activity returns active G subunit to its inactive state, in which it is associated with GDP and the dimeric beta-gamma subunit.
The transduction of the signal results in the production of second messenger molecules. Once produced, the second messengers have a wide variety of effects on cellular activities. One such activity is the activation of cyclic nucleotide-gated (CNG) channels by the cyclic nucleotides cAMP and cGMP. CNG channels are membrane spanning molecules that control the flux of cations through the cellular membrane. The channels are activated—opened—by increased intracellular concentrations of cyclic nucleotide. Once opened the channels conduct mixed cation currents, including ions of Na+, K+, Mg2+ and Ca2+, for example. The activity of the CNG channels couples electrical excitation and Ca2+ signaling to changes in the intracellular concentration of cyclic nucleotides (FIG. 1).
Receptor function is regulated by the G protein itself (GTP-bound form is required for coupling), by phosphorylation (by G-protein-coupled receptor kinases or GRKs) and by binding to inhibitory proteins known as β-arrestins (Lefkowitz, J Biol Chem, 273:18677-18680, 1998). It has long been established that many medically significant biological processes are mediated by proteins participating in signal transduction pathways that involve G proteins and/or second messengers (Lefkowitz, Nature, 351:353-354, 1991). In fact, nearly one-third of all prescription drugs are GPCR ligands (Kallal et al., Trends Pharmacol Sci, 21:175-180, 2000).
GPCRs fall into three major classes (and multiple subclasses) based on their known (or predicted) structural and functional properties (Rana et al., Ann Rev Pharmacol Toxicol, 41:593-624, 2001; Marchese et al., Trends Pharmacol Sci, 20:370-375, 1999). Most of these receptors fall into class A, including receptors for odorants, light, and biogenic amines, for chemokines and small peptides, and for several glycopeptide/glycoprotein hormones. Class B receptors bind higher molecular weight hormones while class C includes GABAB receptors, taste receptors, and Ca2+-sensing receptors. GPCRs are found in all tissues. However, expression of any individual receptor may be limited and tissue-specific. As such some GPCRs may be used as markers for specific tissue types.
As might be expected from the wide range of GPCRs and GPCR ligands, aberrant function of these molecules has been implicated in a large number of human disease states (Rana et al. and Ji et al., supra). GPCR agonists and antagonists have been developed to treat many of these diseases. For example the important group of receptors for biogenic amines has been the target of a large number of successful drugs. Among the receptors in this group are those for epinephrine and norepinephrine (α- and β-adrenergic receptors), dopamine, histamine, and serotonin. Examples of diseases in which GPCR function has been implicated include, but are by no means limited to: heart disease (e.g. tachycardia, congestive heart failure, etc.), asthma, hypertension, allergic reactions (including anaphylactic shock), gastrointestinal disorders, and a wide range of neurological disorders (e.g. Parkinson's disease, depression, schizophrenia, etc.). Finally, many receptors for drugs of abuse are GPCRs.
In many animals, GPCRs are found throughout the organism and are responsible for the maintenance of normal function as well as for pathological conditions. In other instances, the expression of specific GPCRs or families of GPCRs is very tightly controlled, e.g., being expressed only during early developmental stages, etc. Consequently, it is important to find compounds that can stimulate or activate GPCRs, or inhibit or deactivate GPCRs as needed. Agonists—compounds that stimulate the normal function of the GPCRs—have been used to treat asthma, Parkinson's disease, acute heart failure, osteoporosis, hypotension, etc. Antagonists, compounds that interfere with or block normal function have been used to treat, hypertension, myocardial infarction, ulcers, asthma, allergies, psychiatric and neurological disorders, anorexia and bulimia.
In addition to well-characterized receptors, many “orphan” receptors have been cloned (Marchese et al., supra) which are known from sequence similarities to be part of these families, but for which no function or ligand(s) have been discerned. Given the central role of GPCRs in control of diverse cellular activities, there remains a need in the art for methods to identify the agonists and antagonists of these “orphan” receptors as well as to identify additional antagonists for those receptors whose agonists—ligands—are known.
As the first recognized second messenger, cAMP is synthesized by adenylate cyclase in response to activation of many receptors coupled to G proteins Gs and Golf and cyclase activity is inhibited by activation of receptors coupled to G protein Gi. cAMP activates cAMP-dependent protein kinase A (PKA) resulting in profound cellular responses. Physiologically, cAMP mediates such hormonal responses as mobilization of stored energy (e.g., the breakdown of carbohydrates in liver or triglycerides in fat cells, conservation of water by kidney, and Ca2+ homeostasis), control of the rate and contraction force of the heart muscle, relaxation of smooth muscle, production of sex hormones, and many other endocrine and neural processes.
There are a number of cAMP assays currently available. They include transcription reporter assay where a luciferase reporter is driven with a cAMP response promoter element CRE, cAMP immunoassay (Applied Biosystems Forster City, Calif.), an in vitro enzymatic assay for adenylyl cyclase (Molecular Devices, Sunnyvale, Calif.) and cAMP fluorescence polarization assay (PerkinElmer Life Sciences, Boston, Mass.). However, all these assays are end point assays where the cells are lysed and extracts are used for the tests. R. Y. Tsien and his colleagues have also developed fluorescent probes that report cAMP levels in single cells. However, the methods of application of these probes to cells makes them not suitable for high throughput screening formats (Adams et al., 1991, Nature 349:694-697; Zoccolo et al., 2000, Nat. Cell Biol. 2:25-29). There is a need in the art to be able to detect the activation of individual living cells for their cAMP production, particularly in a heterogeneous cell or tissue environment. Such detection capability would further allow the examination of receptor activation and cellular response to complex stimuli, as in the case of induced long-term memory. There also exists in the art a need for the ability to directly examine the cAMP in live cells in order to identify ligands for orphan GPCRs based on the concurrent examination of both Ca2+ and cAMP activation in a given cell as well as to identify agents that modulate GPCR-mediated activity. These and other needs are met by the present invention.